The present invention relates to a fast NMR (nuclear magnetic resonance) imaging method, and more particularly to an image reconstruction method based on an echo-planar method or the like.
The echo-planar method is described in J. Phys. C: Solid State Physics, Vol. 10, pp. L55--L58, 1977. In this method, the amplitude G.sub.X of an oscillating field gradient and the amplitude G.sub.Y of a steadily applied field gradient satisfy the relation of EQU G.sub.X =2MG.sub.Y ( 1)
where M is the size of an image matrix. (The image matrix includes M.times.M pixels.) It is assumed that the dimensions of each of a field of view and a pixel in x and y directions are equal to each other.
It is said that the order of 0.2 gauss/cm is usually required for G.sub.Y. Provided that M is 128, 51.2 gauss/cm is required for G.sub.X. However, the switching of the field gradient having such a large amplitude is practically almost impossible.
In order to solve the above-mentioned problem, a method called a fast Fourier imaging is proposed in Magnetic Resonance in Medicine, Vol. 2, pp. 203-217, 1985.
In the echo-planar method, an image can be reconstructed in principle by one signal measurement subsequent to the application of a 90.degree. pulse. On the other hand, in a generally used Fourier imaging, M measurements are required. The fast Fourier imaging method is a compromise between the echo-planar method and the Fourier imaging. Namely, in the fast Fourier imaging method, though N measurements satisfying a relation of 1.ltoreq.N.ltoreq.M are required, the requirement on the amplitudes of field gradients corresponding to the equation (1) is moderated or ##EQU1## In the case of the above-mentioned numerical example for G.sub.Y and M, 1.6 gauss/cm is required for G.sub.X when N is 32. It is practically difficult even to realize this value of G.sub.X. In order to realize a practical range of G.sub.X &lt;1 gauss/cm, N must be selected to be equal to or greater than 64 in the above-mentioned numerical example for G.sub.Y and M. The value of 64 for N is only a half of 128 which is the number of times of measurement in the Fourier imaging.
As an example most relevant to the present invention is known a method described in the article written by the present inventors and reported in Magnetic Resonance in Medicine, Vol. 5, pp. 485-491, 1987. In this method, each of the amplitude and frequency of an oscillating field gradient can be reduced to half by making an image reconstruction through the simultaneously combined use of a group of echoes obtained when the field gradient is positive and a group of echoes obtained when the field gradient is negative. However, in this method, since a division process is introduced in the course of calculation, there is a drawback of the increase in noise in a final reconstructed image. This increase in noise is discussed in detail in the above-mentioned article.